The AD621 is an instrumentation amplifier with two pins where you can put a jumper or resistor to set the gain. Open we get a gain of 10. Shorted we get a gain of 100. Or we can insert a resistor to get a gain between 10 and 100. Two formulas on the data sheet are suggested. One calculated a resistor value for a desired gain. The other calculates a gain for a given resistor. The problem is I can't get these two formulas to agree. Data sheet is attached. It won't let me attach a spreadsheet. Has anybody got these to work?.